This invention relates to certain metal salts of lower alkyl phosphorodithioic acids, and, in particular, to such metal salts which are oil-soluble. Such metal salts are useful as antioxidants and antiwear agents particularly in lubricating oil compositions employed in the lubrication of internal combustion engines.
Metal salts of phosphorodithioic acids have been used widely as lubricant additives for inhibiting corrosion and improving extreme pressure properties. Obviously, for these purposes, oil-soluble materials are required. In addition to being oil-soluble to the extent required in a functional lubricating oil, it is preferred and desirable that the metal salts of phosphorodithioic acids be sufficiently oil-soluble to prepare oil concentrates of the salts. Oil concentrates may contain as much as 90% of the metal salts. One of the known techniques for improving the oil solubility of metal phosphorodithioates is to incorporate alkyl groups into the phosphorodithioic acid which contain a sufficient number of carbon atoms to provide oil solubility.
The procedures for preparing phosphorodithioic acids and their metal salts are well known in the art. The acids are prepared, for example, by the reaction of phosphorus pentasulfide with an alcohol or a phenol. The reaction involves 4 moles of the alcohol or phenol per mole of phosphorus pentasulfide and may be carried out within a temperature range of from about 50.degree. C. to about 200.degree. C. Hydrogen sulfide is liberated, and the residue is a defined acid. The preparation of the metal salt of phosphorodithioic acids may be effected by reaction of the acid with a metal neutralizing agent such as zinc, zinc oxide, barium oxide, etc. Simply mixing and heating these reactants is sufficient to cause the neutralization to take place, and the resulting product is sufficiently pure for use as a lubricant additive. Phosphorodithioic acids also have been prepared from mixtures of alcohols, and one advantage of the use of a mixture of alcohols is that the lower molecular weight, and expensive alcohols can be utilized in combination with the higher molecular weight, more expensive alcohols, and metal salts prepared from many such acids are oil-soluble.
Various suggestions have been made in the prior art regarding the nature, type, and carbon content of the alkyl or aryl groups present in dialkylphosphorodithioic acids used to prepare desired metal salts. For example, U.S. Pat. No. 2,344,393 taught that it was previously recognized that metal dithiophosphates should have one or more long chain alkyl groups to render them sufficiently soluble in lubricating oils to be of practical value. The patentees found, however that the zinc salt of diamylphosphorodithioic acid was oil-soluble. U.S. Pat. No. 3,318,808 discloses that the higher carbon containing alkyl groups (above 4 carbon atoms) enhance oil solubility. Thus, the patent teaches combinations of C.sub.4 and lower primary and/or secondary alcohols with C.sub.5 and above alcohols, and the ratio of the alcohols is selected to suit the balance between economics and solubility.
U.S. Pat. No. 3,190,833 describes oil-soluble metal phosphorodithioates which are the salts of metals in Group II of the periodic table and comprise preferably the salts of calcium, barium, strontium, zinc and cadmium with phosphorodithioic acids which contain a total of at least about 7.6 aliphatic carbon atoms per atom of phosphorus. To improve the oil solubility of the metal salts, they are reacted with up to about 0.75 mole of an epoxide.
Another patent which relates to the preparation of phosphorodithioic acid salts as useful additives in lubricants is U.S. Pat. No. 3,000,822. This patent describes zinc salts of a mixture of dialkylphosphorodithioic acids wherein the alkyl groups comprise a mixture of lower molecular weight primarily aliphatic hydrocarbon radicals having less than 5 carbon atoms and higher molecular weight primary aliphatic hydrocarbon radicals having at least 5 carbon atoms. The mole ratios of lower molecular weight radicals to higher molecular weight radicals in the zinc salt is within the range of 1:1 to 3:1.
Various suggestions have been made in the prior art for improving the utility of lower alkyl phosphorodithioic acid salts which have a tendency to be oil insoluble. U.S. Pat. No. 4,306,984 describes a procedure for rendering oil insoluble metal C.sub.2 -C.sub.3 dialkyldithiophosphates oil-soluble by forming a complex between the dithiophosphate and an alkenyl or alkyl mono- or bis-succinimide. This combination of additives is used in lubricating oils which can be employed for crankcase lubrication of internal combustion engines. Another method which has been suggested for preparing non-crystalline mixtures of basic or mixed basic and neutral zinc salts of dialkyldithiophosphates containing from 1 to 13 carbon atoms in the alkyl groups has been suggested in U.S. Pat. No. 3,843,530. The mixtures of basic or mixed basic and neutral zinc salts described in this patent contain from 4 to 13 different alkyl groups, have an average carbon content of 3.5 to 4.5, and contain at least 12% by weight of zinc.